The Duke and Duchess of Cambridge. Image credit: www.princeofwales.gov.uk

One of the world's highest profile couples - William and Kate (the Duke and Duchess of Cambridge) - have announced that they are expecting a child.

Due to Britain's constitutional changes, the child will become an heir to the crown, regardless of gender. The baby will be third in line to the throne, after its grandfather (Prince Charles) and father, moving Prince Harry to fourth in line.